This invention relates to a polarization diversity heterodyne receiving device of a baseband combining type.
Such a receiving device is for use in an optical communication network in receiving an incident signal or optical beam through an optical fiber with an incident beam intensity. While transmitted through the optical fiber, the incident signal beam is subjected to disturbances or turbuleneces to be received by the receiving device with an incident state of polarization which complicatedly fluctuates from time to time.
In the manner which will later be described with reference to the accompanying drawing, a polarization diversity heterdyne receiving device comprises an optical heterodyne receiving arrangement for receiving the incident signal beam to produce first and second intermediate frequency (IF) signals having first and second signal amplitudes, respectively. When the receiving device is of a baseband combining type, a demodulating arrangement is used to first demodulate the first and the second intermediate frequency signals into first and second baseband signals, respectively. A signal combiner is used to subsequently combine the first and the second baseband signals into a device output signal having an output amplitude which is independent of the incident state of polarization.
The heterodyne receiving arrangement comprises a coupling and splitting device for coupling and splitting the incident signal beam and a local oscillation beam into first and second optical beams including first and second components of the incident signal beam which are orthogonally polarized components, namely, have orthogonal planes of polarization. An optical detecting device is used in detecting the first and the second optical beams to produce the first and the second intermediate frequency signals.
In the manner described in U.S. patent application Ser. No. 291,885 filed Dec. 29, 1988, by the present inventor et al (European Patent Application No. 88 12 1791.3 filed Dec. 28, 1988). the hetorodyne receiving arrangement may be of a balanced receiver type. Alternatively, the coupling and splitting arrangement may comprise a beam coupler, such as a directional optical coupler, for coupling the incident signal beam and the local oscillation beam into a coupled beam. A polarization beam splitter is used to split the coupled beam into the first and the second optical beams.
In a polarization diversity heterodyne receiving device of the baseband combining type, the first and the second baseband signals have first and second baseband amplitudes, respectively, which are related to the first and the second signal amplitudes of the first and the second intermediate frequency signals, respectively. It is possible to make the receiving device have a least degraded reception sensitivity despite heterodyne reception when the first and the second baseband amplitudes can be rendered quadratically proportional to, namely, proportional to squares of, the first and the second signal amplitudes, respectively. Combination of these baseband signals into the device output signal is called the square-law combination in the art. In practice, the square-law combination is realized by making each of demodulators of the demodulating arrangement have a quadratic characteristic.
In general, an optical signal receiving device must comprise an automatic gain control facility for use in compensating for an undesirable fluctuation in the incident beam intensity. In an optical heterodyne receving device in particular, an optical detector is used in first detecting the incident signal beam to produce an intermediate frequency signal having a signal intensity. Subseqently, a demodulator is used to demodulate the intermediate frequency signal into the device output signal. Usually, the demodulator has a considerably narrow dynamic range in which the signal intensity is variable. It is therefore necessary to compensate for a variation in the signal intensity of the intermediate frequency signal supplied to the demodulator. In the optical hetorodyne receiving device, gain control is carried out to make the intermediate frequency signal have a constant intensity and thereby to compensate for the fluctuation in the incident beam intensity.
This manner of compensation is not applicable to a polarization diversity heterodyne receiving device of the baseband combining type because of the following facts. In the manner described hetetobefore, the first and the second intermediate frequency signals are individually demodulated. Each of the intermediate frequency signal has a signal intensity dependent on a combination of the incident beam intensity and the incident state of polarization. It is objectionable to demodulate the intermediate frequency signals with only the signal intensities rendered constant with no attention directed to the fluctuation in the state of polarization. If supplied with such intermediate frequency signals, the demodulators would produce the baseband signals, each with its baseband amplitude kept constant. This makes it impossible to carry out the square-law combination. As a result, the reception sensitivity is seriously degraded.